Embodiments of the present disclosure relate to an accelerometer device comprises an optical fiber arranged to be deformable in response to a component of an acceleration event in a direction of a desired sensitivity, deformation of the optical fiber varying the length thereof, which is indicative of an acceleration. Embodiments of the present disclosure also relate to a method of sensing an acceleration, for example, a method where an optical fiber deforms in response to a component of an acceleration event in a direction of desired sensitivity, the deformation varying the length of the optical fiber, which is indicative of an acceleration.
In the field of oilfield services, it is known to use sensors in order to monitor acceleration as part of a monitoring program for boreholes and when deploying underwater sensing cables. In various environments, it is desirable to measure acceleration induced by acoustic wavefields. For example, it is known to bury Ocean Bottom Cables (OBCs) beneath the seabed in order to monitor various parameters associated with a reservoir, such monitoring being known as permanent reservoir monitoring. In another example, it is known to deploy towed streamers for hydrocarbon discovery, and it is desirable to measure accelerations induced by acoustic wavefields in respect of each streamer.
Traditionally, the accelerometers employed for such applications rely upon an electronic principle of operation, for example the so-called piezoelectric effect. However, it may be desirable to employ a passive solution in the form of fiber-optic sensors, for reasons of, for example, power efficiency and size. Many such passive solutions are based upon the principle that a winding of optical fiber, when deformed, changes its length and the change in length can be measured. Furthermore, the deformation can be achieved through use of a mass contacting the optical fiber in response to an acceleration event, the mass causing the optical fiber to deform outwardly from within the winding.
Different optical devices and methods exist for measuring acceleration. For example, U.S. Pat. No. 7,243,543 relates to a so-called highly sensitive accelerometer. The accelerometer comprises a lozenge-shaped former about which an optical fiber is coiled. A mass is disposed within the coil and moveable in the direction of the winding of the coil. However, due to the limitations imposed by locating the mass within the coil, the sensitivity of detection of acceleration may be limited and the overall dimensions of the accelerometer may be undesirably large for deployment in a cable.
U.S. Pat. No. 8,499,638 describes to a fiber-optic accelerometer and a method of manufacturing a fiber-optic accelerometer. Such accelerometers comprise a module for making measurements in a given axis. The module includes a coil of optical fiber and a translatable mass disposed within the coil. The mass is disposed at a slant angle relative to a central axis of the coil. However, the design of the module limits the size of the mass and so limits the sensitivity of the module.
U.S. Pat. No. 7,222,534 relates to an optical accelerometer, optical inclinometer and seismic sensor system comprising a beam having at least one optical fiber affixed to a side of the beam. However, a three component accelerometer formed using the optical accelerometers described therein is of a size that is incompatible with insertion in a towed streamer or an ocean bottom cable. Indeed, for ocean bottom cables, the three component accelerometer would need to be housed in a node on the seabed.
U.S. Pat. No. 8,079,261 discloses to an accelerometer having a compliant cylindrical member over which optical fiber is coiled. A piston-like inner mass is disposed within the cylindrical member, but having a shoulder that engages an end of the cylindrical member so that axial movement of the mass results in deformation of the compliant cylindrical member and so deformation of the optical fiber. However, the design of the accelerometer results in a somewhat bulky device for measuring accelerations in respect of three dimensions of a coordinate system. Additionally, the use of a compliant material having a Young's modulus such that it is capable of axial compression under low levels of loading may result in an accelerometer lacking in longevity; this would be particularly disadvantageous for ocean bottom applications.